My desk looks like it belongs to a Red Bull-swilling stock trader, with at least three screens working at once. One screen is for email, one is for my calendar, and the main screen is for everything else. I'd take this setup with me everywhere if it were possible. Thanks to the the new Duet Display iOS app ($15), combined with a Lightning cable, and my iPad Air 2, I now have a workable second screen from my MacBook Air.
Duet Display turns your iPad or iPhone into a second screen for your Mac. Setup is extremely easy. You just download the app onto your iOS device; install the Mac software client (free); connect the two devices with a Lightning or 30-pin adapter cable; and off you go.
Duet Display was developed by former Apple engineers. Though it's not exactly unique, it beats Air Display, which I reviewed back in 2012, in one critical way. Air Display uses a shared Wi-Fi network to connect your iPad with your Mac or Windows computer. The problem is that, when on a public Wi-Fi network, I often had trouble getting Air Display to work.
At the time, an Air Display support person responded to my problems with this message: "Air Display needs certain network settings opened in order to connect, and public networks often have these disabled for security reasons … Another problem is that the responsiveness of the app is almost entirely based on the speed and stability of your wireless network. These public networks are often very slow when you are able to connect to them."
In other words, Duet Display hardwires your two devices together so it doesn't share those connectivity issues.
Duet Display bills itself as being "lag free," but in my tests, that wasn't the case. Some YouTube videos skipped slightly but noticeably. There was an occasional lag when moving the cursor around, both on my iPad as well as on my connected MacBook Air. Touch navigation on my iPad didn't always work as intended, which I guess is understandable, because the Mac OS isn't designed for touchscreen operations. Also, after I closed the Duet Display iOS app and reopened it, I had to fiddle around a bit to reestablish the connection with my MacBook Air. Then there's the $15 price tag, which is a lot for an app.
To sum that up: If you use Macs and iPads, Duet Display is a relatively inexpensive and reasonably functional solution for second (or third) screen addicts.